A single French Islamic State jihadi has emerged as the link among at least four plots to attack France since June, three people with knowledge of the investigation said.

The precise role of the extremist, Rachid Kassim, is under investigation, but the officials say he has become a key instigator who directs recruits in encrypted forums on how and where to carry out the Islamic State's call for European Muslims to strike at home.

Most recently, he was believed to be in contact with a 19-year-old in an unprecedented cell of French women who failed in their attempts to detonate a car bomb and kill police.

From the Loire River town of Roanne, the 29-year-old Kassim is believed to be in either Syria or Iraq yet figures in multiple French anti-terror investigations.

Kassim's virtual fingerprints were found as early as the June 14 knifing of two police officials at their home in the Paris suburb of Magnanville, in which the killer left behind not only a video that he had streamed on Facebook Live but a hit list of politicians, journalists and public personalities. That list is believed to have been drawn up by Kassim ahead of time, one of the officials said.

Attacker Larossi Abballa was shot dead when police stormed the home to rescue the dead couple's three-year-old child. His video, although removed swiftly from Facebook, reappeared in Islamic State propaganda.

Kassim then became the public face of Islamic State's claim of responsibility for the July 14 truck attack on Nice that left 86 people dead, appearing in a video in which he threatened France and beheaded a captive in Iraq.

On July 26, two Frenchmen who came together on the encrypted app Telegram slit the throat of an elderly priest in the Normandy town of Saint-Etienne-du-Rouvray, holding nuns and a couple of parishioners hostage until police fatally shot the attackers.

In that case, too, Kassim appeared to have a role, namely in bringing the two - Adel Kermiche and Abdel Malik Petitjean - together. Again, selfie videos filmed by the men were almost immediately transmitted to Islamic State's media organisations.

And this past weekend, a 15-year-old boy was arrested at his Paris home to thwart what authorities feared was a planned weekend attack - yet another person under investigation for possible links to Kassim, who was known even before he left to join Islamic State for proselytising among young people.

Islamic State has claimed responsibility in its official media for both the attack on the church and against the police. It has remained silent in the case of the car loaded with gas cylinders and the subsequent stabbing of two police officers, which the Paris prosecutor said came during a raid to prevent the women from carrying out another planned attack.

One of those women, identified as 23-year-old Sarah H, was engaged first to Abballa and then to Kermiche, according to the prosecutor, Francois Molins. Her current betrothed, Mr Molins said, is the brother of a man arrested in the Magnanville case, yet another thread in the web linking the cases.

"The act by these young women remote-controlled by individuals who can be found in Syria among the ranks of Daesh demonstrates that this organisation wants to make women into combatants," said Mr Molins, using another name for the Islamic State group.

Kassim himself seemed to confirm that fear in an encrypted Telegram message, according to Le Monde newspaper, which did not say how it acquired the message.

"You have to understand, if women are taking action it's certainly because too few men will take action," he wrote, seemingly referring to the youngest in the female cell, a 19-year-old who was identified through her father's abandoned car.

"She had her whole life before her but she left, she left to do something and they stopped her before she could succeed. But I'm talking about her intention. You, what's your excuse?"